Planets have undergone significant bombardment by comets and planetesimals in the past and in fact, as the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 showed, these impacts are an ongoing process in the evolution of planetary atmospheres. During such an impact, in addition to introducing new material to the planet from the planetesimal itself, such as "soot" and trace species (discussed in Chapters 4 and 5), a small fraction of the planet's atmosphere may be driven off if it acquires sufficient kinetic energy to escape the planet's gravity.
We can make a simple calculation of the rate at which the atmosphere is ejected in this way. Suppose the impactor has a radius R and speed vs and strikes an atmosphere of density aa per unit area above the level where the impactor's energy is spent. The kinetic energy acquired by the air through which the comet passes may be estimated to be roughly
Assuming all this energy is converted to eject a mass Me of atmosphere at the escape velocity ve, we can obtain an upper estimate of atmospheric mass loss kR 2aav 2
This process, unlike thermal escape and hydrodynamic escape, does not discriminate with respect to mass and hence does not cause fractionation of species. However, its effect on the evolution of giant planet atmospheres is likely to be very small due to the large escape velocities of these planets.
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